1. Field of the Invention
The invention relates to the maintenance of the arc in a high intensity discharge lamp during normal and emergency operation, as when there is an instability or a failure in the A-C power normally supplied to the ballast powering the lamp.
2. General Background of the Invention
High intensity discharge (HID) and other arc discharge lamps exhibit the undesirable characteristic of an extinguished arc which renders the lamp useless immediately after a momentary instability or failure in the AC power supplying the lamp. Should the normal, stable power supplying such a lamp be interrupted for as little as a four millisecond period, the arc of the lamp will extinguish. Following the extinguishing of an arc in such as a HID lamp, the remaining high temperature of the arc tube and high pressure of the gas within the tube render the restriking of the arc difficult without extraordinary means incorporated into the lamp and its ballast. This delay of the ability to restrike the lamp and have it reach full lumen output after unwanted arc extinguishing may last as long as 15 minutes.
One means of solving the restrike problem is to incorporate means within the lamp to develop a very high voltage pulse from a special igniter within the lamp. U.S. Pat. Nos. 4,355,261; 5,801,494; and 5,909,082 and the several patents referenced therein illustrate various approaches to incorporating starting and restarting aids into arc discharge lamps to provide a rapid restart in the event of an unscheduled, unwanted shut-down of the arc as by an instability in the AC supply to the lamp. While this style of accommodation to the undesirable time delay in restarting of such as an HID lamp is generally effective in getting the arc and lamp back on line, there are adverse effects upon the lamp. In general, the illustrated restart systems impose adverse high voltages and attendant high pressures within the arc tube, causing additional stress on the lamp components which cause additional wear and tear on the lamp and the likelihood of a premature failure or end of life cycle.
Alternative means to avoid a need to restrike the lamp arc include using an inverter running simultaneously with the AC ballast (as by being powered by an alternative power source, such as a battery) such that when the AC power fails, the inverter continues to run, maintaining power to the ballast and the arc during any mains AC power instability or interruption. The inclusion of such an alternative inverter may be as a means for just supplying a stabilized arc current in the event the normal supply fails or becomes unstable. Alternatively, the additional circuitry may include means for generating a specially stabilized waveform for powering the lamp, either continually or on demand. Means for specially stabilized waveforms for arc current supply have evolved in the growing utilization of higher frequency current supplies for HID lamps. Increased operating frequencies allow the use of lighter weight and smaller size transformers incorporated into the supply/control components for operating HID lamps, however, with these advantages, it has been observed that the lamps are now much more sensitive to variations in supply current/waveform and are subject to developing acoustic resonances in the internal arc current which can cause the lamp to fail. U.S. Pat. Nos. 4,612,478: 4,713,553; 4,928,038; 5,569,984; 5,900,701; and 6,0775,326 and the several patents referenced therein describe the problems to be overcome and the solutions propounded.
The present invention is distinguished from the prior art cited above in that it is a supplemental solution which operates in conjunction with the normal operating system (ballast) which operates the lamp. As such, it is easily incorporated into a HID ballast unit or as an add-on to an existing lamp system by merely being connected to the normal lamp arc supply circuit in a manner analogous to the connecting of an external emergency ballast to a fluorescent lamp. In its function, the present invention does not offer an alternative power supply to operate the lamp, i.e., supply power to the HID lamp ballast rather, it functions only to maintain the arc current during the momentary instability or failure of the regular AC supply. By thus maintaining the arc within the lamp, albeit at a significantly reduced output value (i.e., level of illumination), when the normal AC supply returns or a standby source such as a generator or UPS system comes on line, the lamp springs immediately to normal or near normal illumination mode, nearly to full rated output since the arc has not become extinguished during this time of instability of supply power. The present invention is adaptable to be included into HID lamps having internal emergency ballasts for maintaining operation for limited periods of mains AC failure as well as those systems utilizing a central battery system or a backup generator. The invention is particularly advantageous in those systems wherein there are momentary, as opposed to prolonged, instabilities in AC supply by virtue of the direct ability to maintain the arc in the lamp through the unstable power supply condition.
A collateral aspect of the present invention includes a waveform sensing and recognition system and method which can identify a changing electrical supply waveform which contains the requisite energy to support arc lighting at rated lumens, however in a varying or varied waveform different than that supplied by normal AC mains (i.e., square wave, half wave, rectified, etc.). The waveform monitoring of the arc maintenance system is thus adaptive in that the supplemental, battery supplied arc sustaining current will be continued for the requisite time during the change wherein a backup power supply might supply such as a square wave or otherwise non-sinusoidal supply, at which time the changed, but now stable waveform will be recognized as stable and capable of operating a lamp unassisted and the arc maintenance ballast returns to standby. The inventive method of waveform sampling and storage for signal monitoring and verification provides a waveform recognition mechanism which adapts to changes in electrical supply power, such as a voltage waveform, by first verifying that the waveform contains sufficient electrical energy to operate the lamp, and the sampling and storing of one full period of a “target” waveform, establishing it as a valid waveform for the stable power supply, and subsequently sampling and evaluating the next successive waveforms as a valid and acceptable supply waveform. Should a subsequently evaluated waveform deviate from the “target”, the included high frequency inverter ballast is immediately energized to supply a “maintenance” current directly to the lamp to maintain the arc. During the period of supply instability or interruption, the arc maintenance current continues, and the waveform monitor continues to sample the supplied voltage waveform (AC or emergency power) to find an acceptable, repeating waveform capable of driving the HID lamp ballast and lamp, i.e., a new target waveform. Once the monitoring system identifies a voltage waveform of sufficient energy value to supply the HID lamp ballast and lamp, and that voltage is stable, as by its cycle repeating a predetermined number of times as compared to the new target waveform, the monitoring system turns off the arc maintenance voltage and the HID lamp operates “normally” from the verified supply. As may be recognized in such a system, the “acceptable” new “target” waveform envelope may be different from the previous “target” waveform, given that energy level and repeatability from cycle to cycle are the criteria, and so long as the compared new target waveform is identified as within the parameters currently selected, i.e., of sufficient “RMS” value to illuminate the lamp and the waveform is recognized as stable (i.e., repeating within the set parameters). In the present invention, the new target waveform conceivably may be a steady-state DC supply. Those skilled in the art may thus appreciate that the inventive waveform monitoring system may recognize as acceptable a waveform for a voltage supply which shifts from such as mains AC sinusoidal in form to that of a square waveform from an auxiliary generator or a battery-type uninterruptible power supply (UPS).
None of the known prior art systems for operating such as an HID lamp or a UPS provide an adaptive system and method for observing an instantaneous acceptable supply waveform which may change dramatically in form, yet be distinguished to be able to successfully supply an electrical apparatus such as an HID lamp. Alternative applications of the waveform monitoring system may incorporated to monitor power supplies to sensitive electrical, and electronic equipment, including computers to provide alarms or other indicia of the instability of the waveform of the power supply. With such embodiments, corrective action may be taken to keep such equipment on line, or merely have an operator made aware of the potential of erratic equipment operation. Alternatively, the invention may be utilized with waveform recording/storage apparatus, such as an oscilloscope, to identify and retain the particular waveform of an input voltage or current so that the irregularities in the waveform may be evaluated. The present invention readily identifies the particular “non-standard” waveform, thereby eliminating an otherwise tedious process of trying to capture these often random events.